1. Field of the Invention
This invention relates generally to spread spectrum communications systems. More specifically, the disclosed apparatus and method relate to carrier recovery and tracking in MPSK-modulated systems.
2. Description of the Related Art
A communication system is designed to transmit data signals from a source to a destination. The communication system usually includes three components: a transmitter, a channel, and a receiver. The transmitter processes (modulates) the information data into a form that it can be transmitted through the channel. The channel provides a physical medium for signal transmission between the transmitter and the receiver. Examples of a channel are coaxial cable, optical fiber, and air. The channel usually distorts the transmitted signal by the addition of noise and time delay as the signal passes through it. The receiver receives the transmitted signal and processes (also known as demodulates) it to obtain the original information data. Since the signal received from a practical channel is usually distorted and delayed, the receiver generally requires mechanisms such as equalization and synchronization to assist in demodulating the data.
One type of communication system is the mobile communication system in which the signal is transmitted in the form of electromagnetic waves and the transmission channel is air. The mobile phone communication system is a typical example of this type of communication system.
Particularly, one type of spread spectrum mobile communication systems is a Code Division Multiple Access (CDMA) system. As described in Mobile Cellular Communications by C. Y. Lee, this type of system operates within a frequency bandwidth much larger than the information bandwidth. Spread spectrum systems perform better in the presence of narrow band noise interference and multiple user interference. A particular CDMA system, a Direct Sequence Code Division Multiple Access (DS-CDMA) system, spreads the data stream in the time domain based upon a given spreading code. Orthogonal spreading codes are often employed among different users to minimize multi-user interference. The received signal at the receiver is correlated with the spreading code so that the desired user signal is enhanced relative to signals from other users. In this way users can share the same time and frequency slot.
Various modulation schemes can be used in DS-CDMA systems. One of the commonly used modulation techniques is Multiple Phase Shift Keying (MPSK), in which the serial data is grouped into log2M-bit symbols. Binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK) are particular techniques of MPSK, where M=2 and M=4, respectively. Each information data bit in a MPSK signal is modulated with a pseudo-noise code (PN code) of length N. The spreaded data symbols are up-converted and transmitted on a carrier frequency, fc.
Generally, the tracking range of an MPSK system is limited by the number of keying phases, M. It would be advantageous to design an MPSK receiver having a tracking range independent of the number of keying phases.
A multiple phase shift keying (MPSK) spread spectrum communications receiver is provided that uses weighted correlation techniques for carrier recovery and tracking. The receiver requires a carrier frequency that matches the frequency of an associated transmitter as well as the sampling (or chip) and symbol clocks that are synchronized with those of the transmitter. In the disclosed receiver, a carrier tracking subsystem continually tracks the carrier frequency of the received signal using a tracking scheme that includes weighted correlation techniques. The weighted correlation technique combines signals from two correlator modules, a conventional correlator module and a weighted correlator module, to generate the correlation output. In the weighted correlator module, each product term of the correlation is weighted by a sampling index within a symbol period and then summed to obtain a weighted correlation value. The receiver has a tracking range that is twice the symbol rate and the tracking range is independent of the number of keying phases M.
One aspect of the spread spectrum communications system includes a transmitter, a receiver, and a channel. The transmitter is configured to transmit an MPSK spread spectrum signal over the channel to the receiver. The receiver includes a carrier frequency tracking module. The carrier frequency tracking module includes a correlator configured to generate weighted correlation values for I-phase and Q-phase channel signals.
Another aspect of the invention includes a spread spectrum communications receiver. The receiver comprises a data demodulation module, a synchronization module, and a carrier frequency tracking module. The carrier frequency tracking module includes a correlator configured to generate weighted correlation values for I-phase and Q-phase channel signals.
Yet another aspect of the invention includes a method for demodulating a spread spectrum communications signal in a receiver. The method comprises the steps of: (1) receiving an MPSK spread spectrum signal; (2) generating a correlation for an I-phase and Q-phase channel; (3) weighting a correlation for an I-phase and Q-phase channel; (4) multiplying the weighted correlation of the I-phase channel with the correlation of the Q-phase channel; (5) multiplying the correlation of the I-phase channel with the weighted correlation of the Q-phase channel; (6) subtracting the results of the multiplication steps to generate an error signal; (7) adjusting a carrier frequency of the receiver; and (8) demodulating the spread spectrum signal.